Methods of oxidizing multiwalled carbon nanotubes

ABSTRACT

Methods of oxidizing multiwalled carbon nanotubes are provided. The multiwalled carbon nanotubes are oxidized by contacting the carbon nanotubes with gas-phase oxidizing agents such as CO 2 , O 2 , steam, N 2 O, NO, NO 2 , O 3 , and ClO 2 . Near critical and supercritical water can also be used as oxidizing agents. The multiwalled carbon nanotubes oxidized according to methods of the invention can be used to prepare rigid porous structures which can be utilized to form electrodes for fabrication of improved electrochemical capacitors.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates broadly to methods of oxidizing the surface ofmultiwalled carbon nanotubes. The invention also encompasses methods ofmaking aggregates of surface-oxidized nanotubes, and using the same. Theinvention also relates to complex structures comprised of suchsurface-oxidized carbon nanotubes linked to one another.

2. Description of the Related Art

Carbon Nanotubes

This invention lies in the field of submicron graphitic carbon fibrils,sometimes called vapor grown carbon fibers or nanotubes. Carbon fibrilsare vermicular carbon deposits having diameters less than 1.0μ,preferably less than 0.5μ, and even more preferably less than 0.2μ. Theyexist in a variety of forms and have been prepared through the catalyticdecomposition of various carbon-containing gases at metal surfaces. Suchvermicular carbon deposits have been observed almost since the advent ofelectron microscopy. (Baker and Harris, Chemistry and Physics of Carbon,Walker and Thrower ed., Vol. 14, 1978, p. 83; Rodriguez, N., J. Mater.Research, Vol. 8, p. 3233 (1993)).

In 1976, Endo et al. (see Obelin, A. and Endo, M., J. of Crystal Growth,Vol. 32 (1976), pp. 335-349), hereby incorporated by reference,elucidated the basic mechanism by which such carbon fibrils grow. Theywere seen to originate from a metal catalyst particle, which, in thepresence of a hydrocarbon containing gas, becomes supersaturated incarbon. A cylindrical ordered graphitic core is extruded whichimmediately, according to Endo et al., becomes coated with an outerlayer of pyrolytically deposited graphite. These fibrils with apyrolytic overcoat typically have diameters in excess of 0.1μ, moretypically 0.2to 0.5μ.

In 1983, Tennent, U.S. Pat. No. 4,663,230, hereby incorporated byreference, describes carbon fibrils that are free of a continuousthermal carbon overcoat and have multiple graphitic outer layers thatare substantially parallel to the fibril axis. As such they may becharacterized as having their c-axes, the axes which are perpendicularto the tangents of the curved layers of graphite, substantiallyperpendicular to their cylindrical axes. They generally have diametersno greater than 0.1μ and length to diameter ratios of at least 5.Desirably they are substantially free of a continuous thermal carbonovercoat, i.e., pyrolytically deposited carbon resulting from thermalcracking of the gas feed used to prepare them. Thus, the Tennentinvention provided access to smaller diameter fibrils, typically 35 to700 Å (0.0035 to 0.070μ) and to an ordered, “as grown” graphiticsurface. Fibrillar carbons of less perfect structure, but also without apyrolytic carbon outer layer have also been grown.

The carbon nanotubes which can be oxidized as taught in thisapplication, are distinguishable from commercially available continuouscarbon fibers. In contrast to these fibers which have aspect ratios(L/D) of at least 10⁴ and often 10⁶ or more, carbon fibrils havedesirably large, but unavoidably finite, aspect ratios. The diameter ofcontinuous fibers is also far larger than that of fibrils, beingalways >1.0μ and typically 5 to 7μ.

Tennent, et al., U.S. Pat. No. 5,171,560, hereby incorporated byreference, describes carbon fibrils free of thermal overcoat and havinggraphitic layers substantially parallel to the fibril axes such that theprojection of said layers on said fibril axes extends for a distance ofat least two fibril diameters. Typically, such fibrils are substantiallycylindrical, graphitic nanotubes of substantially constant diameter andcomprise cylindrical graphitic sheets whose c-axes are substantiallyperpendicular to their cylindrical axis. They are substantially free ofpyrolytically deposited carbon, have a diameter less than 0.1μ andlength to diameter ratio of greater than 5. These fibrils can beoxidized by the methods of the invention.

When the projection of the graphitic layers on the nanotube axis extendsfor a distance of less than two nanotube diameters, the carbon planes ofthe graphitic nanotube, in cross section, take on a herring boneappearance. These are termed fishbone fibrils. Geus, U.S. Pat. No.4,855,091, hereby incorporated by reference, provides a procedure forpreparation of fishbone fibrils substantially free of a pyrolyticovercoat. These carbon nanotubes are also useful in the practice of theinvention.

Carbon nanotubes of a morphology similar to the catalytically grownfibrils described above have been grown in a high temperature carbon arc(Iijima, Nature 354, 56, 1991). It is now generally accepted (Weaver,Science 265, 1994) that these arc-grown nanofibers have the samemorphology as the earlier catalytically grown fibrils of Tennent. Arcgrown carbon nanofibers after colloquially referred to as “bucky tubes”,are also useful in the invention.

Carbon nanotubes differ physically and chemically from continuous carbonfibers which are commercially available as reinforcement materials, andfrom other forms of carbon such as standard graphite and carbon black.Standard graphite, because of its structure, can undergo oxidation toalmost complete saturation. Moreover, carbon black is amorphous carbongenerally in the form of spheroidal particles having a graphenestructure, carbon layers around a disordered nucleus. The differencesmake graphite and carbon black poor predictors of nanotube chemistry.

Aggregates of Carbon Nanotubes and Assemblages

As produced carbon nanotubes may be in the form of discrete nanotubes,aggregates of nanotubes or both.

Nanotubes are prepared as aggregates having various morphologies (asdetermined by scanning electron microscopy) in which they are randomlyentangled with each other to form entangled balls of nanotubesresembling bird nests (“BN”); or as aggregates consisting of bundles ofstraight to slightly bent or kinked carbon nanotubes havingsubstantially the same relative orientation, and having the appearanceof combed yarn (“CY”) e.g., the longitudinal axis of each nanotube(despite individual bends or kinks) extends in the same direction asthat of the surrounding nanotubes in the bundles; or, as, aggregatesconsisting of straight to slightly bent or kinked nanotubes which areloosely entangled with each other to form an “open net” (“ON”)structure. In open net structures the extent of nanotube entanglement isgreater than observed in the combed yarn aggregates (in which theindividual nanotubes have substantially the same relative orientation)but less than that of bird nest.

The morphology of the aggregate is controlled by the choice of catalystsupport. Spherical supports grow nanotubes in all directions leading tothe formation of bird nest aggregates. Combed yarn and open nestaggregates are prepared using supports having one or more readilycleavable planar surfaces, e.g., an iron or iron-containing metalcatalyst particle deposited on a support material having one or morereadily cleavable surfaces and a surface area of at least 1 squaremeters per gram. Moy et al., U.S. application Ser. No. 08/469,430entitled “Improved Methods and Catalysts for the Manufacture of CarbonFibrils”, filed Jun. 6, 1995, hereby incorporated by reference,describes nanotubes prepared as aggregates having various morphologies(as determined by scanning electron microscopy).

Further details regarding the formation of carbon nanotube or nanofiberaggregates may be found in the disclosure of U.S. Pat. No. 5,165,909 toTennent; U.S. Pat. No. 5,456,897 to Moy et al.; Snyder et al., U.S.patent application Ser. No. 07/149,573, filed Jan. 28, 1988, and PCTApplication No. US89/00322, filed Jan. 28, 1989 (“Carbon Fibrils”) WO89/07163, and Moy et al., U.S. patent application Ser. No. 413,837 filedSep. 28, 1989 and PCT Application No. US90/05498, filed Sep. 27, 1990(“Battery”) WO 91/05089, and U.S. application Ser. No. 08/479,864 toMandeville et al., filed Jun. 7, 1995 and U.S. application Ser. No.08/284,917, filed Aug. 2, 1994 and U.S. application Ser. No. 08/320,564,filed Oct. 11, 1994 by Moy et al., all of which are assigned to the sameassignee as the invention here and are hereby incorporated by reference.

Nanotube mats or assemblages have been prepared by dispersing nanofibersin aqueous or organic mediums and then filtering the nanofibers to forma mat or assemblage. The mats have also been prepared by forming a gelor paste of nanotubes in a fluid, e.g. an organic solvent such aspropane and then heating the gel or paste to a temperature above thecritical temperature of the medium, removing the supercritical fluid andfinally removing the resultant porous mat or plug from the vessel inwhich the process has been carried out. See, U.S. patent applicationSer. No. 08/428,496 entitled “Three-Dimensional Macroscopic Assemblagesof Randomly Oriented Carbon Fibrils and Composites Containing Same” byTennent et al., which has issued as U.S. Pat. No. 5,691,054 on Nov. 25,1997, hereby incorporated by reference.

Oxidation of Fibrils

McCarthy et al., U.S. patent application Ser. No. 08/329,774 filed Oct.27, 1994, hereby incorporated by reference, describes processes foroxidizing the surface of carbon fibrils that include contacting thefibrils with an oxidizing agent that includes sulfuric acid (H₂SO₄) andpotassium chlorate (KClO₃) under reaction conditions (e.g., time,temperature, and pressure) sufficient to oxidize the surface of thefibril. The fibrils oxidized according to the processes of McCarthy, etal. are non-uniformly oxidized, that is, the carbon atoms aresubstituted with a mixture of carboxyl, aldehyde, ketone, phenolic andother carbonyl groups.

Fibrils have also been oxidized non-uniformly by treatment with nitricacid. International Application PCT/US94/10168 filed on Sep. 9, 1994 asWO95/07316. discloses the formation of oxidized fibrils containing amixture of functional groups. Hoogenvaad, M. S., et al. (“MetalCatalysts supported on a Novel Carbon Support”, Presented at SixthInternational Conference on Scientific Basis for the Preparation ofHeterogeneous Catalysts, Brussels, Belgium, September 1994) also foundit beneficial in the preparation of fibril-supported precious metals tofirst oxidize the fibril surface with nitric acid. Such pretreatmentwith acid is a standard step in the preparation of carbon-supportednoble metal catalysts, where, given the usual sources of such carbon, itserves as much to clean the surface of undesirable materials as tofunctionalize it.

In published work, McCarthy and Bening (Polymer Preprints ACS Div. ofPolymer Chem. 30 (1)420(1990)) prepared derivatives of oxidized fibrilsin order to demonstrate that the surface comprised a variety of oxidizedgroups. The compounds they prepared, phenylhydrazones,haloaromaticesters, thallous salts, etc., were selected because of theiranalytical utility, being, for example, brightly colored, or exhibitingsome other strong and easily identified and differentiated signal. Thesecompounds were not isolated and are, unlike the derivatives describedherein, of no practical significance.

Fisher et al., U.S. Ser. No. 08/352,400 filed Dec. 8, 1994, Fisher etal., U.S. Ser. No. 08/812,856 filed Mar. 6, 1997, Tennent et al., U.S.Ser. No. 08/856,657 filed May 15, 1997, Tennent, et al., U.S. Ser. No.08/854,918 filed May 13, 1997 and Tennent et al., U.S. Ser. No.08/857,383 filed May 15, 1997 all hereby incorporated by referencedescribe processes for oxidizing the surface of carbon fibrils thatinclude contacting the fibrils with a strong oxidizing agent such as asolution of alkali metal chlorate in a strong acid such as sulfuricacid.

Additionally, these applications also describe methods of uniformlyfunctionalizing carbon fibrils by sulfonation, electrophilic addition todeoxygenated fibril surfaces or metallation. Sulfonation of the fibrilscan be accomplished with sulfuric acid or SO₃ in vapor phase which givesrise to carbon fibrils bearing appreciable amounts of sulfones so muchso that the sulfone functionalized fibrils show a significant weightgain. U.S. Pat. No. 5,346,683 to Green, et al. describes uncapped andthinned carbon nanotubes produced by reaction with a flowing reactantgas capable of reacting selectively with carbon atoms in the capped endregion of arc grown nanotubes.

U.S. Pat. No. 5,641,466 to Ebbesen et al. describes a procedure forpurifying a mixture of arc grown arbon nanotubes and impurity carbonmaterials such as carbon nanoparticles and possibly amorphous carbon byheating the mixture in the presence of an oxidizing agent at atemperature in the range of 600° C. to 1000° C. until the impuritycarbon materials are oxidized and dissipated into gas phase.

In a published article Ajayan and Iijima (Nature 361, p. 334-337 (1993))discuss annealing of carbon nanotubes by heating them with oxygen in thepresence of lead which results in opening of the capped tube ends andsubsequent filling of the tubes with molten material through capillaryaction.

In other published work, Haddon and his associates ((Science, 282, 95(1998) and J. Mater. Res., Vol. 13, No. 9, 2423 (1998)) describetreating single-walled carbon nanotube materials (SWNTM) withdichlorocarbene and Birch reduction conditions in order to incorporatechemical functionalities into SWNTM. Derivatization of SWNT with thionylchloride and octadecylamine rendered the SWNT soluble in common organicsolvents such as chloroform, dichlororomethane, aromatic solvents andCS₂.

Functionalized Nanotubes

Functionalized nanotubes have been generally discussed in U.S. Ser. No.08/352,400 filed on Dec. 8, 1994 and in U.S. Ser. No. 08/856,657 filedMay 15, 1997, both incorporated herein by reference. In theseapplications the nanotube surfaces are first oxidized by reaction withstrong oxidizing or other environmentally unfriendly chemical agents.The nanotube surfaces may be further modified by reaction with otherfunctional groups. The nanotube surfaces have been modified with aspectrum of functional groups so that the nanotubes could be chemicallyreacted or physically bonded to chemical groups in a variety ofsubstrates.

Complex structures of nanotubes have been obtained by linking functionalgroups on the tubes with one another by a range of linker chemistries.

Representative functionalized nanotubes broadly have the formula[C_(n)H_(L)—]R_(m)

-   -   where n is an integer, L is a number less than 0.1 n, m is a        number less than 0.5n,    -   each of R is the same and is selected from SO₃H, COOH, NH₂, OH,        O, CHO, CN, COCl, halide, COSH, SH, R′, COOR′, SR′, SiR′₃,        SiOR′R′_(3-y), SiO—SiR′₂OR′, R″, Li, AIR′₂, Hg—X, TIZ₂ and        Mg—X,    -   y is an integer equal to or less than 3,    -   R′ is alkyl, aryl, heteroaryl, cycloalkyl aralkyl or        heteroaralkyl,    -   R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl        or cycloaryl,    -   X is halide, and    -   Z is carboxylate or trifluoroacetate.

The carbon atoms, C_(n), are surface carbons of the nanofiber.

There are many drawbacks associated with the methods now available toprovide oxidized carbon nanotubes. For example, one disadvantage ofusing strong acid treatment is the generation of environmentally harmfulwastes. Treating such wastes increases the production costs of theproducts in which oxidized nanotubes can be used, such as electrodes andcapacitors.

It would, therefore, be desirable to provide methods of oxidizing carbonnanotubes which do not use or generate environmentally hazardouschemicals, and which can be scaled up easily and inexpensively.

While many uses have been found for carbon nanotubes and aggregates ofcarbon nanotubes, as described in the patents and patent applicationsreferred to above, many different and important uses may still bedeveloped if the nanotubes surfaces are oxidized. Oxidation permitsinteraction of the oxidized nanotubes with various substrates to formunique compositions of matter with unique properties and permitsstructures of carbon nanotubes to be created based on linkages betweenthe functional sites on the surfaces of the carbon nanotubes.

OBJECTS OF THE INVENTION

It is, therefore, a primary object of this invention to provide methodsof oxidizing multiwalled carbon nanotubes having a diameter no greaterthan 1 micron.

It is a further and related object to provide methods of oxidizingmultiwalled carbon nanotubes by utilizing environmentally benignoxidizing agents such as CO₂, O₂, steam, H₂O, No, NO₂, O₃ and ClO₂.

It is a further object to provide methods of producing a network ofmultiwalled carbon nanotubes oxidized by the methods of the invention.

It is still a further object to provide methods for preparing rigidporous structures from oxidized multiwalled nanotubes.

It is still a further object to provide an electrochemical capacitorhaving at least one electrode prepared from multiwalled carbon nanotubesoxidized according to methods of the invention.

SUMMARY OF THE INVENTION

The present invention, which addresses the needs of the prior artprovides methods of oxidizing multiwalled carbon nanotubes having adiameter no greater than 1 micron.

More specifically, it has now been found that multiwalled nanotubes canbe oxidized by contacting them with a gas-phase oxidizing agent atdefined temperatures and pressures. The gas-phase oxidizing agents ofthe invention include CO₂, O₂, steam, N₂O, NO, NO₂, O₃, ClO₂ andmixtures thereof. Near critical and supercritical water can also be usedas oxidizing agents. The oxidized multiwalled carbon nanotubes preparedaccording to methods of the invention include carbon and oxygencontaining moieties, such as carbonyl, carboxyl, aldehyde, ketone,hydroxy, phenolic, esters, lactones and derivatives thereof.

The multiwalled carbon nanotubes oxidized according to methods of thepresent invention can be subjected to a secondary treatment step wherebythe oxygen containing moieties of the oxidized nanotubes react withsuitable reactants to add at least a secondary group onto the surface ofthe oxidized nanotubes.

As a result of the present invention multiwalled carbon nanotubesoxidized according to methods of the invention are provided which arealso useful in preparing a network of carbon nanotubes, a rigid porousstructure or as starting material for electrodes utilized inelectrochemical capacitors.

Electrochemical capacitors assembled from electrodes made from theoxidized multiwalled carbon nanotubes of the invention exhibit enhancedelectrochemical characteristics, such as specific capacitance.

Other improvements which the present invention provides over the priorart will be identified as a result of the following description whichsets forth the preferred embodiments of the present invention. Thedescription is not in any way intended to limit the scope of the presentinvention, but rather only to provide a working example of the presentpreferred embodiments. The scope of the present invention will bepointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a quartz reactor used to carry outgas phase oxidation.

FIG. 2 is an SEM micrograph illustrating aggregates of multiwalledcarbon nanotubes oxidized according to the invention at ×3000magnification.

FIG. 3 is an SEM micrograph illustrating aggregates of multiwalledcarbon nanotubes oxidized according to the invention at ×50,000magnification.

FIG. 4 is an SEM micrograph illustrating aggregates of multiwalledcarbon nanotubes oxidized according to the invention at ×10,000magnification.

FIG. 5 is an SEM micrograph illustrating the tip portion of an aggregateof multiwalled carbon nanotubes oxidized according to the invention at×50,000 magnification.

FIGS. 6A to 6C are each a complex-plane impedance plot, a Bode impedanceplot, and a Bode angle plot, respectively, recorded from anelectrochemical capacitor fabricated from electrodes prepared frommultiwalled carbon nanotubes oxidized according to methods of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The terms “nanotube”, “nanofiber” and “fibril” are used interchangeably.Each refers to an elongated hollow structure having a cross section(e.g. angular fibers having edges) or a diameter (e.g. rounded) lessthan 1 micron. The term “nanotube also includes “buckytubes”, andfishbone fibrils.

“Multiwalled nanotubes” as used herein refers to carbon nanotubes whichare substantially cylindrical, graphitic nanotubes of substantiallyconstant diameter and comprise cylindrical graphitic sheets or layerswhose c-axes are substantially perpendicular to their cylindrical axis,as also described in U.S. Pat. No. 5,171,560 to Tennent, et al..

The term “functional group” refers to groups of atoms that give thecompound or substance to which they are linked characteristic chemicaland physical properties.

A “functionalized” surface refers to a carbon surface on which chemicalgroups are adsorbed or chemically attached.

“Graphenic” carbon is a form of carbon whose carbon atoms are eachlinked to three other carbon atoms in an essentially planar layerforming hexagonal fused rings. The layers are platelets only a few ringsin diameter or they may be ribbons, many rings long but only a few ringswide.

“Graphenic analogue” refers to a structure which is incorporated in agraphenic surface.

“Graphitic” carbon consists of grapheric layers which are essentiallyparallel to one another and no more than 3.6 angstroms apart.

The term “aggregate” refers to a dense, microscopic particulatestructure comprising entangled carbon nanotubes.

The term “micropore” refers to a pore which has a diameter of less than2 nanometers.

The term “mesopore” refers to pores having a cross section greater than2 nanometers and less than 50 nanometers.

The term “surface area” refers to the total surface area of a substancemeasurable by the BET technique.

The term “accessible surface area” refers to that surface area notattributed to micropores (i.e., pores having diameters or cross-sectionsless than 2 ram).

The term “isotropic” means that all measurements of a physical propertywithin a plane or volume of the structure, independent of the directionof the measurement, are of a constant value. It is understood thatmeasurements of such non-solid compositions must be taken on arepresentative sample of the structure so that the average value of thevoid spaces is taken into account.

The term “physical property” means an inherent, measurable property,e.g., surface area, resistivity, fluid flow characteristics, density,porosity, and the like.

The term “relatively” means that ninety-five percent of the values ofthe physical property when measured along an axis of, or within a planeof or within a volume of the structure, as the case may be, will bewithin plus or minus 20 percent of a mean value.

The term “substantially” means that ninety-five percent of the values ofthe physical property when measured along an axis of, or within a planeof or within a volume of the structure, as the case may be, will bewithin plus or minus ten percent of a mean value.

The terms “substantially isotropic” or “relatively isotropic” correspondto the ranges of variability in the values of physical properties setforth above.

The term “predominantly” has the same meaning as the term“substantially”.

Methods of Oxidizing Carbon Nanotubes and Aggregates of Carbon Nanotubes

The present invention provides methods of oxidizing the surface ofcarbon nanotubes. The resulting oxidized nanotubes can be easilydispersed in both organic and inorganic solvents, and especially inwater. The surface-oxidized nanotubes obtained by the methods of thepresent invention can be placed in matrices of other materials, such asplastics, or made into structures useful in catalysis, chromatography,filtration systems, electrodes, capacitors and the like.

The carbon nanotubes useful for the methods of the present inventionhave been more specifically described above under the heading “CarbonNanotubes,” and they are preferably prepared according to U.S.application Ser. No. 08/459,534 filed Jun. 2, 1995 assigned to HyperionCatalysis International, Inc. of Cambridge, Mass., incorporated hereinby reference.

The carbon nanotubes preferably have diameters no greater than onemicron, more preferably no greater than 0.2 micron. Even more preferredare carbon nanotubes having diameters between 2 and 100 nanometers,inclusive. Most preferred are carbon nanotubes having diameters between3.5 and 75 nanometers, inclusive.

The nanotubes are substantially cylindrical, graphitic carbon fibrils ofsubstantially constant diameter and are substantially free ofpyrolytically deposited carbon. The nanotubes include those having alength to diameter ratio of greater than 5 with the projection of thegraphite layers on the nanotubes extending for a distance of at leasttwo nanotube diameters. Most preferred are multiwalled nanotubes asdescribed in U.S. Pat. No. 5,171,560 to Tennent, et al, incorporatedherein by reference.

The methods of the invention include contacting the carbon nanotubeswith a gas-phase oxidizing agent under conditions sufficient to oxidizethe surface of the carbon nanotubes, and especially the external sidewalls of the carbon nanotubes.

Compounds useful as gas-phase oxidizing agents are commercially readilyavailable and include carbon dioxide, oxygen, steam, N₂O, NO, NO₂,ozone, ClO₂ and mixtures thereof In a preferred embodiment the gas-phaseoxidizing agents can be diluted with inert gases such as nitrogen, noblegases and mixtures thereof. The dilution reduces the partial pressure ofthe oxidant to the range of 1 to 760 torr.

Suitable conditions for oxidizing the carbon nanotubes of the inventioninclude a temperature range from about 200° C. to about 600° C. wheneverthe oxidizing agent is oxygen, ozone, N₂O, NO, NO₂, ClO₂ or mixturesthereof. The mass molecular weight of the oxidizing agents of thepresent invention does not exceed 70 g/mole. When the oxidizing agent iscarbon dioxide or steam, the treatment of the carbon nanotubes with thegas-phase oxidizing agent is preferably accomplished in a temperaturerange from about 400° C. to about 900° C. Useful partial pressures ofthe oxidizing agent for the methods of the present invention includecontacting of the carbon nanotubes with the gas-phase oxidizing agentsof the invention in a range from about 1 torr to about 10 atm or 7600torr, preferably 5 torr to 760 torr.

In one aspect of the invention the gas-phase oxidizing agent is nearcritical or supercritical water. Supercritical water refers to waterabove its critical temperature of 374° C. At this temperature, retentionof a condensed phase requires a pressure in excess of 3200 psia. It iswell known that supercritical water exhibits anomalously low viscosity,thus enabling it to penetrate aggregates.

In the vicinity of the critical point, viscosity, and, in fact, most ofthe thermodynamic and transport properties of a compound, correlate withspecific volume. Viscosities useful in practicing the invention can alsobe achieved in near critical water having a specific volume up to twiceits critical specific volume of 0.05 ft3/lb or up to 0.10 ft3/lb. Whilethis range of specific volumes can be achieved by various combinationsof near critical temperature and pressure, at saturation, thiscorresponds to a temperature of 363° C. and a pressure of 3800 psia.

A useful period of time for contacting of the carbon nanotubes oraggregates of carbon nanotubes, with the gas-phase oxidizing agents ofthe invention is from 0.1 hours to about 24 hours, preferably from about1 hour to about 8 hours, and most preferably for about 24 hours.

The present invention provides economical, environmentally benignmethods to oxidize the surface of the multiwalled carbon nanotubes.While not wishing to be bound by theory, it is believed that whentreating the carbon nanotubes with the oxidizing agents of the inventionoxygen-containing moieties are introduced onto the surface side walls ofthe carbon nanotubes. The oxidized nanotubes include moieties such ascarbonyl, carboxyl, aldehyde, phenol, hydroxy, esters, lactones andmixtures thereof. Specifically excluded are moieties in which oxygen isnot directly bonded to carbon. For example, the use of SO₃ vapor resultsin the sulfonation of the carbon nanotubes whereby sulfur containingmoieties are introduced onto the surface of the nanotubes. Sulfonatednanotubes exhibit a significant weight gain by comparison tonon-sulfonated carbon nanotubes.

It has been unexpectedly found that upon treatment with the oxidizingagents of the present invention, the oxidized nanotubes have experienceda weight loss rather than gain. For example, it has been found that thecarbon nanotubes experienced a weight loss from about 1% to about 60% byweight and preferably from about 2% to about 15% by weight by comparisonto the unoxidized carbon nanotubes.

While it is not intended to be bound by theory, it has been wellestablished that the edge carbon of a graphite sheet is much moresusceptible to chemical reaction than the basal plane carbon. The carbonnanotubes useful in the present invention have a tubular structureresembling buckytubes. On the surface along the axis, the carbon atomshave the characteristics of basal plane graphite except for thoseassociated with defect sites. However, the carbon atoms at the end of ananotube are either edge carbons or carbons associated with high-energybonds, like members of a five-carbon ring or atoms attached to acatalyst particles. All of these carbons are much more susceptible tochemical attack. Thus, upon treatment with the oxidizing agents of theinvention the nanotubes may become shortened and surface carbon layersmay be partially stripped.

The oxidized nanotubes produced by the methods of the invention exhibitupon titration an acid titer of from about 0.05 meq/g to about 0.6 meq/gand preferably from about 0.1 meq/g to about 0.4 meq/g. For example, thecontent of carboxylic acid is determined by reacting an amount of 0.1 NNaOH in excess of the anticipated titer with the sample and then backtitrating the resulting slurry with 0.1N HCl to an end point determinedpotentiometrically at pH7.

Another aspect of the invention relates to treating aggregates of carbonnanotubes with the gas-phase oxidizing agents. The aggregates treatedaccording to the invention display a macromorphology which can bedescribed as “loose bundles” having the appearance of a severelyweathered rope. The nanotubes themselves retain a morphology similar tothe as-synthesized nanotubes, however, with oxygen-containing moietiesattached to the nanotube surfaces. While it is not intended to be boundby theory, it is believed that in the case of aggregates, the chemicalbonding between the catalyst plate which defines the size of the bundlesand the nanotubes is eliminated. In addition, the nanotubes may exhibitshortening and carbon layers are believed to become partially stripped.An increase in specific surface area has also been observed. Forexample, untreated aggregates have a specific surface area of about 250m²/gm, while oxidized aggregates display a specific surface area up to400 m²/gm. The foregoing changes which take place upon oxidiation withthe oxidizing agents of the invention have been observed by scanningelectron microscopy (SEM), transmission electron microscopy (TEM) andsurface area measurement. SEM photographs shown in FIGS. 2-5 taken aftertreatment of aggregates with the oxidizing agents of the inventionsupport the structured changes of the nanotubes discussed above.Specifically, FIGS. 3 and 5 show many shortened and separated nanotubeends which can be seen at the ends of and on the surface of the “loosebundles” of oxidized nanotube aggregates. More importantly, FIGS. 3 and5 also show that the macrostructure or macromorphology and averagediameters of the aggregates (combed yarn in FIG. 5) remain almostunchanged by comparison to those of the unoxidized aggregates.Similarly, the oxidized aggregates retain the original loose powder formof the unoxidized aggregates. As a result of the addition of oxygencontaining moieties on the surface of oxidized carbon nanotubes and theretention of their macrostructure, the gas-phase oxidized nanotubesobtained by methods of the invention have shown increased dispersion inpolar solvents.

Gas phase oxidized nanotubes can also be used in the production of highquality extrudates which can be formed by using a small amount of watersoluble binder. In the preparation of extrudates, the oxidized surfaceof the nanotubes allows for improved binder dispersion during the mixingstage and minimizes the segregation of binder in the subsequent heatingstep.

Secondary Derivatives of Oxidized Nanotubes

Advantageously, the oxidized nanotubes obtained by the oxidizing methodsof the invention can be further treated. In one embodiment of theinvention after the oxidized nanotubes are formed, they may be furthertreated in a secondary treatment step, by contacting with a reactantsuitable to react with moieties of the oxidized nanotubes thereby addingat least another secondary functional group. Secondary derivatives ofthe oxidized nanotubes are essentially limitless. For example, oxidizednanotubes bearing acidic groups like —COOH are convertible byconventional organic reactions to virtually any desired secondary group,thereby providing a wide range of surface hydrophilicity orhydrophobicity.

The secondary group that can be added by reacting with the moieties ofthe oxidized nanotubes include but are not limited to alkyl/aralkylgroups having from 1 to 18 carbons, a hydroxyl group having from 1 to 18carbons, an amine group having from 1 to 18 carbons, alkyl aryl silaneshaving from 1 to 18 carbons and fluorocarbons having from 1 to 18carbons. Other appropriate secondary groups that can be attached to themoieties present on the oxidized nanotubes include a protein, a peptide,an enzyme, an antibody, a nucleotide peptide, an oligonucleotide, anantigen or an enzyme substrate, enzyme inhibitor or the transition stateanalog of an enzyme substrate.

Other Structures

The invention is also in methods for producing a network of carbonnanotubes comprising treating carbon nanotubes with a gas phaseoxidizing agent of the invention for a period of time sufficient tooxidize the surface of the carbon nanotubes, contacting the oxidizedcarbon nanotubes with a reactant suitable for adding a secondaryfunctional group to the surface of the carbon nanotube, and furthercontacting the secondarily treated nanotubes with a cross-linking agenteffective for producing a network of carbon nanotubes. A preferredcross-linking agent is a polyol, polyamine or polycarboxylic acid. Auseful polyol is a diol and a useful polyamine is a diamine.

In one aspect of the invention a network of carbon nanotubes is obtainedby first oxidizing the as-produced carbon nanotubes with the gas-phaseoxidizing agents of the invention, followed by subjecting the oxidizednanotubes to conditions which foster crosslinking. For example, heatingthe oxidized nanotubes in a temperature range from 180° C. to 450° C.resulted in crosslinking the oxidized nanotubes together withelimination of the oxygen containing moieties of the oxidized nanotubes.

The invention also includes three-dimensional networks formed by linkingthe surface-modified nanotubes of the invention. These complexes includeat least two surface-modified nanotubes linked by one or more linkerscomprising a direct bond or chemical moiety. These networks compriseporous media of remarkably uniform equivalent pore size. They are usefulas adsorbents, catalyst supports and separation media.

Three Dimensional Structures

The oxidized nanotubes of the invention are more easily dispersed inaqueous media than unoxidized nanotubes. Stable, porous 3-dimensionalstructures with meso- and macropores (pores>2 nm) are very useful ascatalysts or chromatography supports. Since nanotubes can be dispersedon an individualized basis, a well-dispersed sample which is stabilizedby cross-links allows one to construct such a support. Surface-oxidizednanotubes are ideal for this application since they are easily dispersedin aqueous or polar media and the oxygen-containing moieties present onthe oxidized nanotubes provide cross-link points. Additionally, theoxygen-containing moieties also provide points to support the catalyticor chromatographic sites. The end result is a rigid, 3-dimensionalstructure with its total surface area accessible with secondary groupsites on which to support the active agent.

Although the interstices between these nanotubes are irregular in bothsize and shape, they can be thought of as pores and characterized by themethods used to characterize porous media. The size of the intersticesin such networks can be controlled by the concentration and level ofdispersion of nanotubes, and the concentration and chain lengths of thecross-linking agents. Such materials can act as structured catalystsupports and may be tailored to exclude or include molecules of acertain size. Aside from conventional industrial catalysis, they havespecial applications as large pore supports for biocatalysts.

Typical applications for these supports in catalysis include their useas a highly porous support for metal catalysts laid down byimpregnation, e.g., precious metal hydrogenation catalysts. Moreover,the ability to anchor molecular catalysts by tether to the support viathe secondary groups combined with the very high porosity of thestructure allows one to carry out homogeneous reactions in aheterogeneous manner. The tethered molecular catalyst is essentiallydangling in a continuous liquid phase, similar to a homogeneous reactor,in which it can make use of the advantages in selectivities and ratesthat go along with homogeneous reactions. However, being tethered to thesolid support allows easy separation and recovery of the active, and inmany cases, very expensive catalyst.

These stable, rigid structures also permits carrying out heretofore verydifficult reactions, such as asymmetric syntheses or affinitychromatography by attaching a suitable enantiomeric catalyst orselective substrate to the support. The rigid networks can also serve asthe backbone in biomimetic systems for molecular recognition. Suchsystems have been described in U.S. Pat. No. 5,110,833 and InternationalPatent Publication No. WO93/19844. The appropriate choices forcross-linkers and complexing agents allow for stabilization of specificmolecular frameworks.

Methods of Preparing Rigid Porous Structures

In one aspect of the invention rigid porous structures are prepared byfirst preparing surface-oxidized nanotubes as described above,dispersing them in a medium to form a suspension, separating the mediumfrom the suspension to form a porous structure, wherein thesurface-oxidized nanotubes are further interconnected to form a rigidporous structure, all in accordance with methods more particularlydescribed in U.S. application Ser. No. 08/857,383 (WBAM Docket No.0064734-0080) entitled “Rigid Porous Carbon Structures, Methods ofMaking, Methods of Using and Products Containing Same” filed on May 15,1997, hereby incorporated by reference.

The hard, high porosity structures can be formed from regular carbonnanotubes or nanotube aggregates, either with or without surfacemodified nanofibers (i.e., surface oxidized nanofibers). In order toincrease the stability of the nanotube structures, it is also possibleto deposit polymer at the intersections of the structure. This may beachieved by infiltrating the assemblage with a dilute solution of lowmolecular weight polymer cement kie., less than about 1,000 MW) andallowing the solvent to evaporate. Capillary forces will concentrate thepolymer at nanotube intersections. It is understood that in order tosubstantially improve the stiffness and integrity of the structure, onlya small fraction of the nanotube intersections need be cemented. Oneembodiment of the invention relates to a method of preparing a rigidporous carbon structure having a surface area greater than at least 100m²/gm, comprising the steps of:

-   -   (a) dispersing a plurality of nanofibers in a medium to form a        suspension; and    -   (b) separating said medium from said suspension to form said        structure,    -   wherein said nanotubes are interconnected to form said rigid        structure of intertwined nanotubes bonded at nanotube        intersections within the structure.

The nanotubes may be uniformly and evenly distributed throughout thestructure or in the form of aggregate particles interconnected to formthe structure. When the former is desired, the nanotubes are dispersedthoroughly in the medium to form a dispersion of individual nanotubes.When the latter is desired, nanotube aggregates are dispersed in themedium to form a slurry and said aggregate particles are connectedtogether with a gluing agent to form said structure.

The medium used may be selected from the group consisting of water andorganic solvents. Preferably, the medium comprises a dispersant selectedfrom the group consisting of alcohols, glycerin, surfactants,polyethylene glycol, polyethylene imines and polypropylene glycol.

The medium should be selected which: (1) allows for fine dispersion ofthe gluing agent in the aggregates; and (2) also acts as a templatingagent to keep the internal structure of the aggregates from collapsingas the mix dries down.

One preferred embodiment employs a combination of polyethylene glycol(PEG) and glycerol dissolved in water or alcohol as the dispersingmedium, and a carbonizable material such as low MW phenol-formaldehyderesins or other carbonizable polymers or carbohydrates (starch orsugar). Once the rigid porous structure has been prepared, it can thenbe oxidized with the oxidizing agents of the invention in preparationfor use in electrochemical capacitors, for example. The oxidation occursin the same pressure and temperature ranges as are used to oxidizenanotubes, aggregates or assemblages of carbon nanotubes.

In another embodiment, if surface oxidized nanotubes are employed, thenanotubes are oxidized prior to dispersing in the medium and areself-adhering forming the rigid structure by binding at the nanotubeintersections. The structure may be subsequently pyrolized to removeoxygen. A useful temperature range is from about 200° C. to about 2000°C. and preferably from about 200° C. to about 900° C.

According to another embodiment, the nanotubes are dispersed in saidsuspension with gluing agents and the gluing agents bond said nanotubesto form said rigid structure. Preferably, the gluing agent comprisescarbon, even more preferably the gluing agent is selected from amaterial that, when pyrolized, leaves only carbon. Accordingly, thestructure formed with such a gluing may be subsequently pyrolized toconvert the gluing agent to carbon.

Preferably, the gluing agents are selected from the group consisting ofcellulose, carbohydrates, polyethylene, polystyrene, nylon,polyurethane, polyester, polyamides and phenolic resins.

According to further embodiments of the invention, the step ofseparating comprises filtering the suspension or evaporating the mediumfrom said suspension.

According to yet another embodiment, the suspension is a gel or pastecomprising the nanotubes in a fluid and the separating comprises thesteps of:

-   -   (a) heating the gel or paste in a pressure vessel tot        temperature above the critical temperature of the fluid;    -   (b) removing supercritical fluid from the pressure vessel; and    -   (c) removing the structure from the pressure vessel.

Isotropic slurry dispersions of nanotube aggregates insolvent/dispersant mixtures containing gluing agent can be accomplishedusing a Waring blender or a kneader without disrupting the aggregates.The nanotube aggregates trap the resin particles and keep themdistributed.

These mixtures can be used as is, or can be filtered to removesufficient solvent to obtain cakes with high nanotube contents (5-20%dry weight basis). The cake can be molded, extruded or pelletized. Themolded shapes are sufficiently stable so that further drying occurswithout collapse of the form. On removing solvent, disperant molecules,along with particles of gluing agent are concentrated and will collectat nanotube crossing points both within the nanotube aggregates, and atthe outer edges of the aggregates. As the mixture is further dried downand eventually carbonized, nanotube strands within the aggregates andthe aggregates themselves are glued together at contact points. Sincethe aggregate structures do not collapse, a relatively hard, veryporous, low density particle is formed.

As set forth above, the rigid, porous structures may also be formedusing oxidized nanotubes with or without a gluing agent. Carbonnanotubes become self-adhering after oxidation. Very hard, dense matsare formed by highly dispersing the oxidized nanotubes (asindividualized strands), filtering and drying. The dried mats havedensities between 1-1.2 g/cc, depending on oxygen content, and are hardenough to be ground and sized by-sieving. Measured surface areas areabout 275 m²/g.

Substantially all the oxygen within the resulting rigid structure can beremoved by pyrolizing the particles at about 600° C. in flowing gas, forexample argon. Densities decrease to about 0.7-0.9 g/cc and the surfaceareas increase to about 400 m²/g. Pore volumes for the calcinedparticles are about 0.9-0.6 cc/g, measured by water absorbtion.

The oxidized nanotubes may also be used in conjunction with a gluingagent. Oxidized nanotubes are good starting materials since they haveattachment points to stick both gluing agents and templating agents. Thelatter serve to retain the internal structure of the particles or matsas they dry, thus preserving the high porosity and low density of theoriginal nanotube aggregates. Good dispersions are obtained by slurryingoxidized nanotubes with materials such as polyethyleneimine cellulose(PEI Cell), where the basic imine functions form strong electrostaticinteractions with carboxylic acid functionalized fibrils. The mix isfiltered to form mats. Pyrolizing the mats at temperatures greater than650° C. in an inert atmosphere converts the PEI Cell to carbon whichacts to fuse the nanotube aggregates together into hard structures. Theresult is a rigid, substantially pure carbon structure, which can thenbe oxidized with the oxidizing agents of the present invention.

Solid ingredients can also be incorporated within the structure bymixing the additives with the nanotube dispersion prior to formation ofthe structure. The content of other solids in the dry structure may bemade as high as fifty parts solids per part of nanotubes.

According to one preferred embodiment, nanotubes are dispersed at highshear in a high-shear mixer, e.g. a Waring Blender. The dispersion maycontain broadly from 0.01 to 10% nanotubes in water, ethanol, mineralspirits, etc.. This procedure adequately opens nanotube bundles, i.e.tightly wound bundles of nanotubes, and disperses the nanotubes to formself-supporting mats after filtration and drying. The application ofhigh shear mixing may take up to several hours. Mats prepared by thismethod, however, are not free of aggregates.

If the high shear procedure is followed by ultrasonication, dispersionis improved. Dilution to 0.1% or less aids ultrasonication. Thus, 200 ccof 0.1% fibrils, for example, may be sonified by a Bronson SonifierProbe (450 watt power supply) for 5 minutes or more to further improvethe dispersion.

To achieve the highest degrees of dispersion, i.e. a dispersion which isfree or virtually free of nanotube aggregates, sonication must takeplace either at very low concentration in a compatible liquid, e.g. at0.001% to 0.01% concentration in ethanol or at higher concentration e.g.0.1% in water to which a surfactant, e.g. Triton X-100, has been addedin a concentration of about 0.5%. The mat which is subsequently formedmay be rinsed free or substantially free of surfactant by sequentialadditions of water followed by vacuum filtration. The mat thus formedcan then be oxidized with the oxidizing agents of the invention underconditions sufficient to form oxidized nanotubes within the mat.

Particulate solids such as MnO₂ (for batteries) and Al₂O₃ (for hightemperature gaskets) may be added to the oxidized nanotube dispersionprior to mat formation at up to 50 parts added solids per part ofnanotubes.

Reinforcing webs and scrims may be incorporated on or in the mats duringformation. Examples are polypropylene mesh and expanded nickel screen.

Electrochemical Capacitors

Carbon nanotubes are electrically conductive. Electrodes and their usein electrochemical capacitors comprising carbon nanotubes and/orfunctionalized carbon nanotubes which have been described in U.S.application Ser. No. 08/856,657 (WBAM Docket No. 0064736-0000) entitled“Graphitic Nanofibers in Electrochemical Capacitors,” filed on May 15,1997 incorporated herein by reference.

Further details about electrochemical capacitors based on catalyticallygrown carbon nanotubes are disclosed in Chumming Niu et al., “High PowerElectrochemical Capacitors based on Carbon Nanotube Electrodes,” inApplied Physics Letters 70(11), pp. 1480-1482, Mar. 17, 1997incorporated herein by reference.

The quality of sheet electrode depends on the microstructure of theelectrode, the density of the electrode, the functionality of theelectrode surface and mechanical integrity of the electrode structure.

The microstructures of the electrode, namely, pore size and sizedistribution determines the ionic resistance of electrolyte in theelectrode. The surface area residing in micropores (pore diameter <2 nm)is considered inaccessible for the formation of a double layer (2). Onthe other hand, distributed pore sizes, multiple-pore geometries (deadend pores, slit pores, cylindrical pores, etc.) and surface propertiesusually give rise to a distributed time constant. The energy stored inan electrode with a distributed time constant can be accessed only withdifferent rates. The rapid discharge needed for pulsed power is notfeasible with such an electrode.

The density of the electrode determines its volumetric capacitance. Anelectrode with density less than 0.4 g/cc is not practical for realdevices. Simply, the low-density electrode will take up too muchelectrolyte, which will decrease both volumetric and gravimetriccapacitance of the device.

The surface of the carbon nanotubes is related to the wetting propertiesof electrodes towards electrolytes. The surface of as-produced,catalytically grown carbon nanotubes is hydrophobic. It has beenunexpectedly found that the hydrophobic surface properties of theas-produced carbon nanotubes can be changed to hydrophilic by treatmentof the as-produced carbon nanotubes or aggregates of carbon nanotubeswith the oxidizing agents of the present invention. It has also beenunexpectedly found that the dispersing properties in water ofsurface-oxidized carbon nanotubes are related to weight loss duringtreatment with such gas-phase oxidizing agents as CO₂, O₂, steam, H₂O,NO₂, O₃, ClO₂ and mixtures thereof. For example, oxidized nanotubesexhibiting a weight loss of about 10% by weight can be easily dispersedin water. It is necessary to oxidize on the surface of the carbonnanotubes to improve their wetting properties for aqueous electrolytes.Furthermore, the capacitance can be increased by further attaching redoxgroups on the surface of the carbon nanotubes.

Finally, the structural integrity of the electrodes is critical toreproducibility and long term stability of the device. Mechanicalstrength of electrodes incorporating carbon nanotubes is determined bythe degree of entanglement of the carbon nanotube and bonding betweencarbon nanotubes in the electrode. A high degree of entanglement andcarbon nanotube bonding can also improve the conductivity, which iscritical to the power performance of an electrode. The specificcapacitance (D.C. capacitance) of the electrodes made from gas-phasetreated fibrils was about 40 F/g.

One aspect of the present invention relates to preparing electrodes andelectrochemical capacitors from surface-oxidized carbon nanotubes.Broadly, as prepared carbon nanotubes have been treated with gas-phaseoxidizing agents of the invention to provide surface oxidized,multiwalled carbon nanotubes which can be used to prepare the electrodesof the invention.

In another aspect of the invention, the oxidized nanotubes can befurther treated with a reactant suitable to react with moieties presenton the oxidized nanotubes to form nanotubes having secondary groups onits surface which are also useful in preparing the electrodes of thepresent invention.

Electrodes are assembled by simple filtration of slurries of the treatednanotubes. Thickness is controlled by the quantity of material used andthe geometry, assuming the density has been anticipated based onexperience. It may be necessary to adjust thickness to getself-supporting felts.

The electrodes are advantageously characterized by cyclic voltammetry,conductivity and DC capacitance measurement.

EXAMPLES

The following examples serve to provide further apprection of theinvention but are not meant in any way to restrict the effective scopeof the invention.

Example 1 Oxidation of Carbon Nanotubes With Gas Phase CO₂

Oxidized carbon nanotubes were prepared by using CO₂ in the gaseousphase. About 10 grams of carbon nanotubes were placed into a reactor asshown in FIG. 1. The reactor was a heated quartz tube having a reactingchamber connected at each end to a side tube. The reacting chamber hadan outside diameter of about 3 inches and each side tube has an outsidediameter of about 1 inch. Between the side tube at the bottom side andthe reacting chamber there was a gas permeable porous quartz plate,which supports a bed of carbon nanotubes prepared as described in U.S.application Ser. No. 08/459,534 filed on Jun. 2, 1995.

A stream of gaseous CO₂ was continuously passed down through the bed ofcarbon nanotubes at a rate of about 120 cc/min for 2 hours at about 800°C.

The degree of oxidation was measured by the weight loss exhibited by thecarbon nanotubes; a weight loss of about 10% was recorded. The carbonnanotubes oxidized in this manner dispersed in water quite easilywhereas they hardly did so prior to treatment with gaseous CO₂.

Example 2 Oxidation of Carbon Nanotubes With Wet-air

Carbon nanotubes were oxidized by using wet air. About 10 grams ofcarbon nanotubes prepared according to U.S. application Ser. No.08/459,534 filed on Jun. 2, 1995 were charged into the reactor describedin Example 1.

Air saturated with water vapor at room temperature was continuouslypassed down through the bed of carbon nanotubes at a rate of about 120cc/min. The temperature of the reactor, measured by a k-typethermocouple positioned inside the bed of carbon nanotubes, was set at530° C. The degree of oxidation was controlled by variation of thereaction duration and monitored by weight loss, compared to the initialweighted unoxidized carbon nanotubes. Three samples with weight lossesof 7.1, 12.4, and 68% corresponding to 4, 5, and 8 hr oxidation,respectively, were prepared.

Example 3 Oxidation of Carbon Nanotubes With Oxygen

Carbon nanotubes are oxidized by using oxygen in the gas phase. About 10grams of carbon nanotubes prepared according to U.S. Ser. No. 08/459,534filed on Jun. 2, 1995 are charged into a reactor as described in Example1.

A stream of gaseous oxygen is continuously passed down through the bedof carbon nanotubes at a rate of about 120 cc/min for 2 hours at 600° C.The temperature of the reactor is measured by a k-type thermocouplepositioned inside the bed of carbon nanotubes. The degree of oxidationis controlled by variation of the reaction duration and monitored byweight loss as compared to the initial weight of unoxidized carbonnanotubes. The resulting weight loss is about 10%. The carbon nanotubesoxidized in this manner disperse in water quite easily whereas theyhardly do so prior to treatment with gaseous oxygen.

Example 4 Oxidation of Carbon Nanotubes With N₂O

Carbon nanotubes are oxidized by using N₂O in the gas phase. About 10grams of carbon nanotubes prepared according to U.S. Ser. No.08/459,534filed on Jun. 2, 1995 are charged into a reactor as described in Example1.

A stream of gaseous N₂O is continuously passed down through the bed ofcarbon nanotubes at a rate of about 120 cc/min for 2 hours at 600° C.The temperature of the reactor is measured by a k-type thermocouplepositioned inside the bed of carbon nanotubes. The degree of oxidationis controlled by variation of the reaction duration and monitored byweight loss as compared to the initial weight of unoxidized carbonnanotubes. The resulting weight loss is about 10%. The carbon nanotubesoxidized in this manner disperse in water quite easily whereas theyhardly do so prior to treatment with gaseous N₂O.

Example 5 Oxidation of Carbon Nanotubes With NO

Carbon nanotubes are oxidized by using NO in the gas phase. About 10grams of carbon nanotubes prepared according to U.S. Ser. No.08/459,534filed on June 2, 1995 are charged into a reactor as described in Example1.

A stream of gaseous NO is continuously passed down through the bed ofcarbon nanotubes at a rate of about 120 cc/min for 2 hours at 600° C.The temperature of the reactor is measured by a k-type thermocouplepositioned inside the bed of carbon nanotubes. The degree of oxidationis controlled by variation of the reaction duration and monitored byweight loss as compared to the initial weight of unoxidized carbonnanotubes. The resulting weight loss is about 10%. The carbon nanotubesoxidized in this manner disperse in water quite easily whereas theyhardly do so prior to treatment with gaseous NO.

Example 6 Oxidation of Carbon Nanotubes With NO₂

Carbon nanotubes are oxidized by using NO₂ in the gas phase. About 10grams of carbon nanotubes prepared according to U.S. Ser. No.08/459,534filed on June 2, 1995 are charged into a reactor as described in Example1.

A stream of gaseous oxygen is continuously passed down through the bedof carbon nanotubes at a rate of about 120 cc/min for 2 hours at 600° C.The temperature of the reactor is measured by a k-type thermocouplepositioned inside the bed of carbon nanotubes. The degree of oxidationis controlled by variation of the reaction duration and monitored byweight loss as compared to the initial weight of unoxidized carbonnanotubes. The resulting weight loss is about 10%. The carbon nanotubesoxidized in this manner disperse in water quite easily whereas theyhardly do so prior to treatment with gaseous NO₂.

Example 7 Oxidation of Carbon Nanotubes With Ozone

Carbon nanotubes are oxidized by using ozone in the gas phase. About 10grams of carbon nanotubes prepared according to U.S. Ser. No.08/459,534filed on June 2, 1995 are charged into a reactor as described in Example1.

A stream of gaseous ozone is continuously passed down through the bed ofcarbon nanotubes at a rate of about 120 cc/min for 2 hours at 600° C.The temperature of the reactor is measured by a k-type thermocouplepositioned inside the bed of carbon nanotubes. The degree of oxidationis controlled by variation of the reaction duration and monitored byweight loss as compared to the initial weight of unoxidized carbonnanotubes. The resulting weight loss is about 10%. The carbon nanotubesoxidized in this manner disperse in water quite easily whereas theyhardly do so prior to treatment with gaseous ozone.

Example 8 Oxidation of Carbon Nanotubes With ClO₂

Carbon nanotubes are oxidized by using ClO₂ in the gas phase. About 10grams of carbon nanotubes prepared according to U.S. Ser. No.08/459,534filed on Jun. 2, 1995 are charged into a reactor as described in Example1.

A stream of gaseous ClO₂ is continuously passed down through the bed ofcarbon nanotubes at a rate of about 120 cc/min for 2 hours at 600° C.The temperature of the reactor is measured by a k-type thermocouplepositioned inside the bed of carbon nanotubes. The degree of oxidationis controlled by variation of the reaction duration and monitored byweight loss as compared to the initial weight of unoxidized carbonnanotubes. The resulting weight loss is about 10%. The carbon nanotubesoxidized in this manner disperse in water quite easily whereas theyhardly do so prior to treatment with gaseous ClO₂.

Example 9 Electrochemical Capacitors Prepared From Carbon NanotubesOxidized With CO₂

0.1 g of oxidized nanotubes as prepared in Example 1 were dispersed indeionized water to form a slurry which was then filtered on a 3.5″diameter filter membrane to form a mat with diameter of about 3.3″. Themat was dried at 120° C. for approximately one hour and heated at 350°C. in air for 4 hr. The final weight was 0.095 g. The disk electrodeswith diameter of 0.5″ were made from the mat and soaked overnight in 38%sulfuiric acid held at approximately 85° C. and then kept in the acidsolution at 25° C. until cell assembly. The electrodes were wettedeasily by the electrolyte. Single cell test devices were fabricated withtwo 38% sulfuric acid saturated electrodes separated by a 0.001″ thickpolymer separator which was also wetted with 38% sulfuric acid. Theequivalent series resistance (E.S.R.) of the test device measured at 1kHz using a fixed frequency meter was 0.0430. The capacitance of thedevice was measured by a constant current discharging method. Thecalculated specific capacitance for the electrode was 40 F/g. Thefrequency response analysis was carried out at d.c. biases of 0V, 0.5Vand 1V with a 10 mV amplitude sinusoidal signal using a Solartron model1250B frequency analyzer driving an EG&G PAR model 273potensiostat/galvonostat.

Example 10 Electrochemical Capacitors Prepared From Wet-air OxidizedNanotubes

Nanotubes oxidized as in example 2 were prepared into an electrodeaccording to the process described in example 3. Three single-cell testelectrochemical capacitors were fabricated from electrodes made from thenanotubes with weight losses of 7.1, 12.4, and 68%, respectively. TableI summarizes properties of these electrodes and test results of thecapacitors made from them. The resistivity of the electrodes wasmeasured using the van der Pauw method, on samples with dimensions of0.5 cm×0.5 cm having four leads attached to their comer edges. Ohmiccontact of the leads to the samples was tested by measuring a linear I-Vcurve.

Scanning Electron Microscope (SEM) studies were carried out with a LEO982 scanning electron microscope equipped with a Schottky field-emissiongun.

Cyclic voltammograms were recorded using an EG&G PAR Model 273Potentiostat/Galvonostar connected to a three-electrode cell consistingof a fibril working electrode, a platinum gauze counter electrode and astandard Ag/AgC1 reference electrode. The electrolyte was 38% sulfuricacid.

The equivalent series resistance (E.S.R.) of the test devices wasmeasured using a fixed frequency RCL meter (Fluke PM6303) at 1 kHz. Thespecific capacitance was measured by a d.c. constant current dischargingmethod.

Impedance analysis was carried out with a Solartron 11250 frequencyresponse analyzer driving an EG&G PAR model 273 Potentiostat/Galvonostatat a dc bias of 0, 0,5 and 1V with 10 mV amplitude sinusoidal signal).

Certain characteristics of the three electrodes made from oxidizednanotubes have been summarized in Table I below. TABLE I Sam- WeightThick- Density Resistivity C_(p) ples Treatment loss (%) ness E.S.R.(g/cc) (Ω-cm) (F/g) C_(p, 1 khz) A Wet air, 4 hr 7.1 0.0016″ 0.074 0.481.8 × 10⁻² 33.14 26.53 B Wet air, 5 hr 12.4 0.0015″ 0.041 0.51 1.5 ×10⁻² 35.85 22 C Wet air, 8 hr 68 0.0016″ 0.036 0.52 2.5 × 10⁻² 35.4424.25E.S.R.-Equivalent series resistanceC_(p)-Specific capacitanceC_(p, 1 kHz)-specific capacitance at 1 kHz.

For all three devices, more than 61% of the stored energy was availablefor use at a frequency of one kHz. The frequency responses of the threedevices were almost identical. FIGS. 6A-C show frequency responseanalysis result of the test device fabricated from sample 1 (Table I).The electrodes functioned like a non-porous, planar electrode. This wasevidenced (FIGS. 6A,-6C) in the complex-plane impedance plots in whichno clear “knee” point was present, and further in the Bode angle plot,up to 10 Hz, showing a near −90° phase angle for an ideal capacitor.

As illustrated by the foregoing description and examples, the inventionhas application in the formulation of a wide variety of oxidizednanofibers.

The terms and expressions which have been employed are used as terms ofdescription and not of limitations, and there is no intention in the useof such terms or expressions of excluding any equivalents of thefeatures shown and described as portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention.

1. A method of oxidizing multiwalled carbon nanotubes having a diameterno greater than 1 micron, said method comprising contacting saidmultiwalled carbon nanotubes with a gas-phase oxidizing agent underconditions sufficient to form oxidized nanotubes.
 2. The method of claim1, wherein said oxidation occurs on the exterior side walls of saidmultiwalled carbon nanotubes.
 3. The method of claim 1, wherein saidoxidation occurs on the surface of said multiwalled carbon nanotubes. 4.The method of claim 1, wherein said diameter of said carbon nanotubes isfrom 2 to 100 nanometers.
 5. The method of claim 1, wherein saiddiameter of said carbon nanotubes is from 3.5 to 75 nanometers.
 6. Themethod of claim 1, wherein said multiwalled carbon nanotubes include atleast a plurality of graphitic layers that are substantially parallel tothe axis of said nanotubes.
 7. The method of claim 1, wherein saidmultiwalled carbon nanotubes are substantially cylindrical, graphiticnanotubes having a length to diameter ratio of greater than 5 and adiameter of less than 0.1 micron.
 8. The method of claim 1, wherein saidmultiwalled carbon nanotubes are substantially cylindrical, free of acontinuous pyrolitically deposited carbon overcoat, the projection ofthe graphite layers on said nanotubes extending for a distance of atleast two nanotube diameters.
 9. The method of claim 1, wherein saidmultiwalled carbon nanotube is a fishbone fibril.
 10. The method ofclaim 1, wherein said multiwalled carbon nanotubes are grown onsupported catalysts.
 11. The method of claim 1, wherein said oxidizednanotubes comprise moieties selected from the group consisting ofcarbonyl, carboxyl, aldehyde, phenolic, hydroxy, esters, lactones andderivatives thereof.
 12. The method of claim 1, wherein said oxidizednanotubes exhibit upon titration an acid titer of from 0.05 to about 0.6meq/g.
 13. The method of claim 1, wherein said oxidized nanotubesexhibit upon titration an acid titer from 0.1 to 0.4 meq/g.
 14. Themethod of claim 1, wherein said oxidized carbon nanotubes exhibit aweight loss of from 1% to 60% by comparison to unoxidized carbonnanotubes.
 15. The method of claim 1, wherein said oxidized nanotubesexhibit a weight loss from 2% to 15% by comparison with said unoxidizedcarbon nanotube.
 16. The method of claim 1, wherein said gas-phaseoxidizing agent is selected from the group consisting of CO₂, O₂, steam,N₂O, NO, NO₂ ,O₃, ClO₂ and mixtures thereof.
 17. The method of claim 1,wherein said gas-phase oxidizing agent is diluted with an inert diluantselected from the group consisting of nitrogen, noble gases and mixturesthereof.
 18. The method of claim 1, wherein said gas-phase oxidizingagent is near critical or supercritical water.
 19. The method of claim1, wherein said oxidizing of said multiwalled carbon nanotubes with saidgas-phase oxidizing agent is performed for a period of time from about0.1 hours to about 24 hours.
 20. The method of claim 1, wherein saidoxidizing of said multiwalled carbon nanotubes with said gas-phaseoxidizing agent is performed for a period of time from about 1 hour toabout 8 hours.
 21. The method of claim 1, wherein said oxidizing of saidmultiwalled carbon nanotubes with said gas-phase oxidizing agent isperformed in a temperature range from about 200° C. to about 600° C. andin a range of partial pressure of said oxidizing agent from about 1 torrto about 7600 torr whenever said gas-phase oxidizing agent is selectedfrom the group consisting of O₂,O₃, N₂O, NO, NO₂, ClO₂ and mixturesthereof.
 22. The method of claim 21, wherein the partial pressure rangeof the gas-phase oxidizing agent is from 5 torr to 760 torr.
 23. Themethod of claim 1, wherein said oxidizing of said multiwalled carbonnanotubes with said gas-phase oxidizing agent is performed in atemperature range from about 400° C. to about 900° C. and in a range ofpartial pressure of the oxidizing agent from about 1 torr to about 7600torr whenever said gas-phase oxidizing agent is CO₂ or steam.
 24. Themethod of claim 23, wherein the partial pressure range of the gas-phaseoxidizing agent is from 5 torr to 760 torr.
 25. The method of claim 1,further comprising a secondary treatment step of said oxidized nanotubeswith a reactant suitable to react with moieties of said oxidizednanotubes thereby adding at least a secondary group onto the surface ofsaid oxidized nanotubes.
 26. The method of claim 25, wherein saidadditional secondary group is selected from the group consisting of analkyl or aryl silane wherein said alkyl has C₁ to C₁₈, said aryl has C₁to C_(18,) an alkyl of C₁ to C₁₈ or an aralkyl group of C₁ to C₁₈, ahydroxyl group of C₁ to C₁₈ and an amine group of C₁ to C₁₈.
 27. Themethod of claim 25, wherein said additional secondary group is afluorocarbon.
 28. The method of claim 1 further comprising dispersingsaid surface-oxidized nanotubes into a liquid medium.
 29. The method ofclaim 28, wherein after being dispersed in said liquid medium, saidoxidized nanotubes are filtered and dried to form a mat.
 30. The methodof claim 29, further comprising heating said mat from 200° C. to 900° C.31. The method of claim 29, further comprising forming said mat into anelectrode.
 32. A method for producing a network of carbon nanotubescomprising the steps of: (a) oxidizing said carbon nanotubes with agas-phase oxidizing agent under conditions sufficient to form oxidizednanotubes; (b) subjecting said oxidized nanotubes to conditionssufficient to cause crosslinking.
 33. The method of claim 32, whereinsaid conditions include heating said oxidized nanotubes in air in atemperature range from 200° C. to 600° C.
 34. The method of claim 29,wherein said conditions include heating said oxidized nanotubes in aninert atmosphere in a temperature range from 200° C. to 2000° C.
 35. Amethod for producing a network of oxidized carbon nanotubes comprisingthe steps of: (a) oxidizing said carbon nanotubes with a gas-phaseoxidizing agent under conditions sufficient to form oxidized nanotubes;(b) treating said oxidized nanotubes with a reactant suitable to reactwith moieties of said oxidized nanotubes thereby adding at least asecondary group onto the surface of said oxidized nanotubes; (c) furthercontacting said nanotubes bearing secondary groups with an effectiveamount of crosslinking agent.
 36. The method of claim 32, wherein saidgas phase oxidizing agent is selected from the group consisting of CO₂,O₂, steam, N₂O, NO, NO₂, O₃, ClO₂ and mixtures thereof.
 37. The methodof claim 35, wherein said crosslinking agent is selected from the groupconsisting of a polyol or polyamine.
 38. The method of claim 37, whereinsaid polyol is a diol and said polyamine is a diamine.
 39. The method ofclaim 32, wherein said oxidized nanotubes comprise moieties selectedfrom the group consisting of carbonyl, carboxyl, aldehyde, ketone,hydroxy, phenolic, esters, lactones and derivatives thereof.
 40. Amethod of treating aggregates of carbon nanotubes which comprisescontacting said aggregates with an gas-phase oxidizing agent underconditions sufficient to oxidize said carbon nanotubes.
 41. The methodof claim 40, wherein said aggregates have a macromorphology resembling ashape selected from the group consisting of bird nests, combed yarn andopen net aggregates.
 42. The method of claim 40, wherein said aggregateparticles have an average diameter of less than 50 microns.
 43. Themethod of claim 40, wherein said carbon nanotubes are substantiallycylindrical with a substantially constant diameter are multiwalledhaving graphitic layers concentric with the nanotube axis and aresubstantially free of pyrolitically deposited carbon.
 44. The method ofclaim 40, wherein said carbon nanotubes are a fishbone fibril.
 45. Themethod of claim 40, wherein said treated aggregates resemble a weatheredrope.
 46. A method for preparing a rigid porous structure comprising thesteps of: (a) oxidizing a multiplicity of multiwalled carbon nanotubesaccording to the method of claim 1 to oxidized nanotubes; (b) dispersingsaid oxidized nanotubes in a medium to form a suspension; (c) separatingsaid medium from said suspension to form a porous structure of entangledoxidized nanotubes wherein said nanotubes are interconnected to form arigid porous structures.
 47. The method of claim 46, wherein saidmultiwalled carbon nanotubes are uniformly and evenly distributedthroughout said structure.
 48. The method of claim 46, wherein saidcarbon nanotubes are in the form of aggregate particles selected fromthe groups consisting of aggregate particles resembling a shape selectedfrom the group consisting of bird nest, combed yarn and open net. 49.The method of claim 46, wherein said oxidized nanotubes are in the formof aggregate particles resembling a weathered rope.
 50. The method ofclaim 46, further comprising heating said suspension in air to atemperature in a range from about 200° C. to about 600° C. therebyforming said rigid porous structure.
 51. The method of claim 46, furthercomprising heating said suspension in an inert gas to a temperature in arange from about 200° C. to about 2000° C. therebyforming said rigidporous structure.
 52. The method of claim 46, wherein said medium iswater or organic solvents.
 53. The method of claim 46, wherein saidmedium comprises a dispersant selected from the group consisting ofalcohols, glycerin, surfactants, polyethylene glycol, polyethyleneimines and polypropylene glycol.
 54. The method of claim 46, whereinsaid suspension further comprises gluing agents selected from the groupconsisting of cellulose, carbohydrate, polyethylene, polystyrene, nylon,polyurethane, polyester, polyamides and phenolic resins.
 55. The methodof claim 46, further comprising the steps of: (a) forming said rigidporous structure into a mat; and (b) forming said mat into an electrode.56. An electrochemical capacitor having at least one electrodecomprising the oxidized carbon nanotubes prepared by the method ofclaim
 1. 57. An electrochemical capacitor having at least one electrodeprepared by a method which comprises the following steps: (a) contactingaggregates of carbon nanotubes with a gas-phase oxidizing agent underconditions sufficient to oxidize said carbon nanotubes; (b) dispersingsaid aggregates of oxidized nanotubes prepared in step (a) in a liquidmedium to form a slurry; (c) filtering and drying said slurry to form amat of oxidized carbon nanotubes; (d) subjecting said mat to conditionssufficient to cause the crosslinking of said oxidized carbon nanotubes.58. The electrochemical capacitor of claim 57, wherein said conditionsof step (d) include heating said mat from 180° C. to 350° C.
 59. Anelectrochemical capacitor having at least one electrode formed by amethod comprising the following steps: (a) dispersing aggregates ofcarbon nanotubes in a liquid medium to form a slurry; (b) filtering anddrying said slurry to form a mat of carbon nanotubes; (c) treating saidmat according to the method of claim 1 under conditions sufficient tooxidize said carbon nanotubes.
 60. The capacitor of claim 55, whereinsaid gas-phase oxidizing agent is selected from the group consisting ofCO₂, O₂, steam, N₂O, NO, NO₂, O₃, ClO₂ and mixtures thereof.